Effect of W on Phase Transformation Kinetics and Its Correlation With Localized Corrosion Resistance for UNS S39274

Previous research in 25Cr Super Duplex Stainless Steels (SDSS) has shown that an optimal W concentration—as an element in solid solution—exists in which W improves localized corrosion resistance. Outside this range W is either ineffective or even detrimental. However the mechanisms by which W improves localized corrosion resistance are unclear. For example debate still exists as to whether W enhances passivity or facilitates repassivation. Furthermore it is even less clear whether W accelerates or retards the precipitation of deleterious phases such as χ– and σ–phase.The objective of this investigation is to elucidate the influence of W on phase transformation kinetics and its corresponding effect on crevice corrosion resistance. Herein two SDSS have been investigated: a W-free (UNS S32750) and a 2.1 wt% W-containing (UNS S39274) grade. First different isothermal heat treatments were performed to force the precipitation of tertiary phases in the microstructure. The alloy microstructure was analyzed and quantified by Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Electron Backscatter Diffraction (EBSD) and the type distribution and volume fraction of deleterious phases quantified.After characterization creviced samples—as per ISO 18070—were exposed to natural seawater extracted from 80 meters depth from the Trondheim Fjord. Creviced samples were exposed under three different conditions; 1) freely exposed at OCP ii) polarized to +300 mVAg/AgCl and iii) polarized to +600 mVAg/AgCl. During the exposure the potential evolution was monitored for all samples while the anodic current was measured for ii) and iii). The critical crevice temperature (CCT) was determined by increasing the solution temperature in 5ºC per week until crevice corrosion initiation was observed. After initiation the temperature was decreased 2.5ºC every 48 hours until repassivation was achieved. The temperature at which crevice corrosion ceased to grow was defined as the critical crevice repassivation temperature (CCRT).

Product Number: 51319-13233-SG
Author: Cristian Torres
Publication Date: 2019